This invention relates to a power amplifier, and particularly to a protective device for preventing a power amplifier for acoustic purpose from being brought into an abnormal condition, for example, generation of an abnormal voltage due to shorted load or the like, or failure due to transient abnormal sound upon turning-on and -off of power.
There has been proposed a protective means for preventing a power amplifier from breaking down due to an abnormal voltage, abnormal sound or the like, by providing a relay between the power amplifier and a loudspeaker and actuating the relay upon abnormal operation of the power amplifier to disconnect a signal path.
However, the relay must cut off a large current and thus becomes large in size. In addition, the proportion of the cost of the relay in the cost of the whole amplifier is large and ranks fourth in the proportions of costs of power transformer, heat sink, output transistor and relay which constitutes the amplifier. The cost of the relay occupies most of the cost of the protective device. In addition, only the use of a relay for cutting off output current is insufficient to protect the whole of the amplifier. That is, when a primary failure occurs due to some cause, for example, initial defect or shortcircuit between pattern conductors, secondary, ternary . . . failures are caused in turn like a chain reaction, and finally a large accident such as the emission of smoke or subsequent fire may occur. Even if such a large accident does not occur, the first failure may extended further. It is desirable to restrict the first failure, if it occurs, such that it will not cause secondary and ternary failures.
In short, the conventional protective means has drawbacks that use of a relay leads to high cost and that secondary and ternary failures cannot be prevented.